The article server is offline, but I assume they found out that prolog rule evaluation depends on the order the rules are presented in the program.

If so, the language they thought they were using (and that they should actually use) is datalog, not prolog.

Datalog has declarative semantics: All facts that are derivable from the base database and the rules will be derived by the interpreter, and it will not add extra hallucinated facts. If that's not true, it's a bug in the runtime, not in the language.

20+ years ago, it was the backend for the business rules engine that processed various logging and monitoring events. The concept was interesting, the performance was terrible, and businesses mostly didn't want to touch it. After I setup clients with a generic set of rules that worked on Prolog facts, most all of my clients were happy to limit their changes to only those fact files.

Sometimes the Biorhythm program on my Apple ][ failed to produce correct answers. But it sure was great for impressing cool hippie chicks.

https://www.youtube.com/watch?v=jYoY1cwAd90

Some applications were discussed in https://news.ycombinator.com/item?id=40994552

Dunno about Prolog, but Datomic uses datalog for its query language, and it’s excellent. Datalog is a subset of Prolog.

Datalog may appear to be a subset, but it is quite distinct semantically.

What is Datalog used for nowadays?

Other than databases, program analysis. The polonius borrow checker in rustc uses datalog internally.

But you can use it for lots of things. Whenever I'm frustrated with graph based tools being slow (like build systems), I run the graph through a datalog engine for comparison. It's usually much, much faster.